Management of Douglas fir and Colorado blue spruce needle cast diseases

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included. 

Because of spring rains, you should be concerned about needle cast diseases, as it does not take much rain to activate spore release and disseminate spores that initiate infections. Rain events even into mid-June can still cause spore showers to rain on susceptible needles.

Needle cast diseases are the main reason for the casting of last year’s needles. If you did not get control of the needle cast diseases last year, your trees may begin to cast those needles soon. You will only stop needles from casting in 2010 if you get the diseases under control right now.

Two events must occur before needle cast disease pathogens begin to infect the needles. First, buds need to break with needles becoming roughly half their final size, and second, the spores of the fungus must be released from last year’s infected needles. This latter event occurs when it rains. So bud break, spore release and rain lead to needle cast infection periods in Douglas fir and blue spruce. Since you can’t really tell when the spores release or if it rained enough for spores to spread far, the best bet is to assume that spores will be releasing about the time buds are breaking and take action by following bud break. The most effective material for control of Swiss and Rhabdocline needle casting disease is chlorothalonil.

To put nature on your side, plant disease resistant and disease tolerant trees every year. The Douglas fir seed source Shuswap is absolutely resistant to Rhabdocline needle cast and therefore, all you have to manage is Swiss needle cast. To get the upper hand on spruce needle cast, plant species of spruce other than Colorado blue spruce which is the most susceptible spruce to Rhizosphaera needle cast. Shifting slowly to these disease resistant or tolerant species will reduce pesticide use in the Great Lakes.

To help you save money, time and the environment, it is important to monitor for the amount of needle cast in your plantings before initiating a spray program. To determine if your trees warrant chemical management, look for signs of the fungal fruiting bodies on the needles. These fruiting bodies are producing the spores that will infect the new needles as they break bud. With a hand lens, scan the discolored needles. For Rhabdocline on Douglas fir, look for needles with elongated brown splotches and on the bottom side of the needle, look for areas where the outer portion of the needle’s epidermis (skin) appears raised. If you see these, then examine two-year-old needles on 50 or more trees scattered throughout your plantation. If you find fruiting bodies on 20 percent or more of the trees, consider treating the plantation. Two sprays will be more effective than one.

In the case of Swiss needle cast of Douglas fir and Rhizosphaeria of spruce, randomly select 20 or more trees scattered around the plantation, and remove three sample branches from each tree. Examine the white rows of stomata on two-year-old needles and if half of the branches have dark fruiting bodies on more than 10 percent of the needles, consider treating the entire plantation.

Important things to consider when monitoring for needle cast diseases

For those of you with selective cuts on your farms, older trees can act as sources of disease for the younger trees. If you do not spray this year because you thought the trees were going to market in the fall, sometimes plans change and those trees can become points of infection. Also, survey the landscape around your plantation for old wind- and fence-row trees that will almost always carry spores of needle casting diseases. Trees from these abandoned plantations may be contributing large amounts of disease to your plantations. We have observed spores blowing more than one-half mile and it probably moves farther than that. So, if you monitor the trees within your fields, make sure you consider these other possible sources of infection.

A good spray program started or continued now will prevent poor looking diseased trees in 2010. At lease two sprays to cover the emerging needles should be considered and a third spray in June will provide added insurance.

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